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上一篇博客《 RocketMQ原理学习--延时消息实现原理》我们介绍了延时消息的实现原理，这篇博客我们看看RocketMQ在消费者消息消费失败的情况下是如何处理的。
正常情况下消费者在消息消费失败时会马上重新消费消息，正常情况下再次消费应该会将消息消费完成的，但是当一次消费消费不成功时我们可以设置让Broker延时一段时间再重新投递消息，是不是看到这就发现和延时消息类似了。
1、示例
当我们的业务没有处理完成时，可以返回ConsumeConcurrentlyStatus.RECONSUME_LATER，让Broker延时再投递消息。
public class PushConsumer {
public static void main(String[] args) throws InterruptedException, MQClientException {
DefaultMQPushConsumer consumer = new DefaultMQPushConsumer("rmq-group");
consumer.setNamesrvAddr("localhost:9876");
//consumer.setInstanceName("rmq-instance2");
consumer.subscribe("TopicA-test", "TagA");
consumer.registerMessageListener(new MessageListenerConcurrently() {
public ConsumeConcurrentlyStatus consumeMessage(
List<MessageExt> msgs, ConsumeConcurrentlyContext context) {
for (MessageExt msg : msgs) {
System.out.println(new String(msg.getBody()));
}
//延时再发送
return ConsumeConcurrentlyStatus.RECONSUME_LATER;
}
});
consumer.start();
System.out.println("Consumer Started.");
}
}2、Broker延时消息处理
Broker的SendMessageProcessor的consumerSendMsgBack方法会处理消费者ack失败的消息，发送回的消息会设置重试Topic，重试Topic的名称默认为："%RETRY%" + ConsumerGroup，然后会设置消息的延时机制，默认级别为3表示10秒，这样再延时投送消息。
private RemotingCommand consumerSendMsgBack(final ChannelHandlerContext ctx, final RemotingCommand request)
throws RemotingCommandException {
final RemotingCommand response = RemotingCommand.createResponseCommand(null);
final ConsumerSendMsgBackRequestHeader requestHeader =
(ConsumerSendMsgBackRequestHeader)request.decodeCommandCustomHeader(ConsumerSendMsgBackRequestHeader.class);
if (this.hasConsumeMessageHook() && !UtilAll.isBlank(requestHeader.getOriginMsgId())) {
ConsumeMessageContext context = new ConsumeMessageContext();
context.setConsumerGroup(requestHeader.getGroup());
context.setTopic(requestHeader.getOriginTopic());
context.setCommercialRcvStats(BrokerStatsManager.StatsType.SEND_BACK);
context.setCommercialRcvTimes(1);
context.setCommercialOwner(request.getExtFields().get(BrokerStatsManager.COMMERCIAL_OWNER));
this.executeConsumeMessageHookAfter(context);
}
SubscriptionGroupConfig subscriptionGroupConfig =
this.brokerController.getSubscriptionGroupManager().findSubscriptionGroupConfig(requestHeader.getGroup());
if (null == subscriptionGroupConfig) {
response.setCode(ResponseCode.SUBSCRIPTION_GROUP_NOT_EXIST);
response.setRemark("subscription group not exist, " + requestHeader.getGroup() + " "
+ FAQUrl.suggestTodo(FAQUrl.SUBSCRIPTION_GROUP_NOT_EXIST));
return response;
}
if (!PermName.isWriteable(this.brokerController.getBrokerConfig().getBrokerPermission())) {
response.setCode(ResponseCode.NO_PERMISSION);
response.setRemark("the broker[" + this.brokerController.getBrokerConfig().getBrokerIP1() + "] sending message is forbidden");
return response;
}
if (subscriptionGroupConfig.getRetryQueueNums() <= 0) {
response.setCode(ResponseCode.SUCCESS);
response.setRemark(null);
return response;
}
String newTopic = MixAll.getRetryTopic(requestHeader.getGroup());
int queueIdInt = Math.abs(this.random.nextInt() % 99999999) % subscriptionGroupConfig.getRetryQueueNums();
int topicSysFlag = 0;
if (requestHeader.isUnitMode()) {
topicSysFlag = TopicSysFlag.buildSysFlag(false, true);
}
TopicConfig topicConfig = this.brokerController.getTopicConfigManager().createTopicInSendMessageBackMethod(
newTopic,
subscriptionGroupConfig.getRetryQueueNums(),
PermName.PERM_WRITE | PermName.PERM_READ, topicSysFlag);
if (null == topicConfig) {
response.setCode(ResponseCode.SYSTEM_ERROR);
response.setRemark("topic[" + newTopic + "] not exist");
return response;
}
if (!PermName.isWriteable(topicConfig.getPerm())) {
response.setCode(ResponseCode.NO_PERMISSION);
response.setRemark(String.format("the topic[%s] sending message is forbidden", newTopic));
return response;
}
MessageExt msgExt = this.brokerController.getMessageStore().lookMessageByOffset(requestHeader.getOffset());
if (null == msgExt) {
response.setCode(ResponseCode.SYSTEM_ERROR);
response.setRemark("look message by offset failed, " + requestHeader.getOffset());
return response;
}
//设置为新的topic名称:"%RETRY%+consumerGroup
final String retryTopic = msgExt.getProperty(MessageConst.PROPERTY_RETRY_TOPIC);
if (null == retryTopic) {
MessageAccessor.putProperty(msgExt, MessageConst.PROPERTY_RETRY_TOPIC, msgExt.getTopic());
}
msgExt.setWaitStoreMsgOK(false);
int delayLevel = requestHeader.getDelayLevel();
int maxReconsumeTimes = subscriptionGroupConfig.getRetryMaxTimes();
if (request.getVersion() >= MQVersion.Version.V3_4_9.ordinal()) {
maxReconsumeTimes = requestHeader.getMaxReconsumeTimes();
}
if (msgExt.getReconsumeTimes() >= maxReconsumeTimes
|| delayLevel < 0) {
newTopic = MixAll.getDLQTopic(requestHeader.getGroup());
queueIdInt = Math.abs(this.random.nextInt() % 99999999) % DLQ_NUMS_PER_GROUP;
topicConfig = this.brokerController.getTopicConfigManager().createTopicInSendMessageBackMethod(newTopic,
DLQ_NUMS_PER_GROUP,
PermName.PERM_WRITE, 0
);
if (null == topicConfig) {
response.setCode(ResponseCode.SYSTEM_ERROR);
response.setRemark("topic[" + newTopic + "] not exist");
return response;
}
} else {
//设置延时级别，这样消息在写入commitlog中会被作为延时消息持久化
if (0 == delayLevel) {
delayLevel = 3 + msgExt.getReconsumeTimes();
}
msgExt.setDelayTimeLevel(delayLevel);
}
MessageExtBrokerInner msgInner = new MessageExtBrokerInner();
msgInner.setTopic(newTopic);
msgInner.setBody(msgExt.getBody());
msgInner.setFlag(msgExt.getFlag());
MessageAccessor.setProperties(msgInner, msgExt.getProperties());
msgInner.setPropertiesString(MessageDecoder.messageProperties2String(msgExt.getProperties()));
msgInner.setTagsCode(MessageExtBrokerInner.tagsString2tagsCode(null, msgExt.getTags()));
msgInner.setQueueId(queueIdInt);
msgInner.setSysFlag(msgExt.getSysFlag());
msgInner.setBornTimestamp(msgExt.getBornTimestamp());
msgInner.setBornHost(msgExt.getBornHost());
msgInner.setStoreHost(this.getStoreHost());
msgInner.setReconsumeTimes(msgExt.getReconsumeTimes() + 1);
String originMsgId = MessageAccessor.getOriginMessageId(msgExt);
MessageAccessor.setOriginMessageId(msgInner, UtilAll.isBlank(originMsgId) ? msgExt.getMsgId() : originMsgId);
//持久化消息，会将消息作为延时消息处理
PutMessageResult putMessageResult = this.brokerController.getMessageStore().putMessage(msgInner);
if (putMessageResult != null) {
switch (putMessageResult.getPutMessageStatus()) {
case PUT_OK:
String backTopic = msgExt.getTopic();
String correctTopic = msgExt.getProperty(MessageConst.PROPERTY_RETRY_TOPIC);
if (correctTopic != null) {
backTopic = correctTopic;
}
this.brokerController.getBrokerStatsManager().incSendBackNums(requestHeader.getGroup(), backTopic);
response.setCode(ResponseCode.SUCCESS);
response.setRemark(null);
return response;
default:
break;
}
response.setCode(ResponseCode.SYSTEM_ERROR);
response.setRemark(putMessageResult.getPutMessageStatus().name());
return response;
}
response.setCode(ResponseCode.SYSTEM_ERROR);
response.setRemark("putMessageResult is null");
return response;
}总结：
对于失败重试消息RocketMQ将其处理为延时消息进行操作，进行延时投递就可以了，设计还是比较牛的。
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